The development of fluid catalyst cracking FCC systems has been an on-going challenge since the early '40's. This challenge has been stimulated considerably by the development of improved catalysts and particularly the crystalline zeolite (alumino-silicate) containing catalyst of acceptable cracking activity.
Berg U.S. Pat. No. 2,684,931 identifies early fluidized solids catalytic cracking and regeneration of catalyst solids in dense fluid bed operations. The catalyst solids are conveyed upwardly in riser conduits with lift gas which discharge into the bottom of the dense fluid catalyst beds used to effect hydrocarbon conversion and regeneration of catalyst particles. The lift gas to the regenerator may be flue gas with the lift gas into the cracking zone selected from a number of different materials such as hydrogen, methane and unsaturated or saturated normally gaseous products of cracking.
Keith U.S. Pat. No. 2,702,267 discloses a hydrocarbon conversion process which includes stripping of the fouled catalyst with regeneration gases comprising hydrogen. This reference relies upon the use of a steam-high purity oxygen mixture to achieve the known water gas shift reactions to effect a partial removal of deposits of catalytic cracking.
Owen U.S. Pat. No. 3,886,060 discloses an arrangement of apparatus for effecting separate two stage riser hydrocarbon conversion in combination with two stage regeneration of the catalyst employing a first upflowing catalyst regeneration operation followed by a downflowing catalyst second stage of regeneration.
Haddad et al U.S. Pat. No. 4,219,407 discloses discharging a catalyst suspension from a riser zone outwardly and downwardly through channel means open in the bottom thereof. The downwardly discharged catalyst particles are directed into an elongated confined restricting zone provided with sloping baffle means and stripping steam inlet means in a bottom portion thereof.
Pulak U.S. Pat. No. 4,010,003 discloses an apparatus arrangement comprising a catalyst upflow regeneration zone of larger diameter dimensions in a lower portion than the upper portion of restricted diameter used to convey suspended catalyst of regeneration horizontally into an adjacent flue gas-catalyst particle relatively large separation zone provided with internal cyclone separation means. The suspension so horizontally conveyed is directed downwardly by baffle means within the separation zone.
Crude oils from which desired liquid fuels are obtained contain a highly diverse mixture of hydrocarbons, sulfur, nitrogen compounds and metal contaminants of nickel, vanadium, iron, copper, arsenic and sodium. The hydrocarbons vary widely in molecular weight and structure with the hydrogen lean complicated molecular structures concentrated in the higher boiling portion of the crude oil boiling above vacuum gas oils. For example, crude oils are known in which 30 to 60%, or more, of the total volume of oil is composed of compounds boiling at a temperature above 650.degree. F. and in which from about 10% to about 30%, or more, of the total volume comprise molecular structure which boil above about 1000.degree. F. or 1025.degree. F.
Residual portions of crude oil comprising gas oils and high boiling molecular structures boiling above 650.degree. F. are unsuitable for inclusion in gasoline and desired light cycle oil products. Therefore, the petroleum refining industry is required to develop economic processes for converting the higher boiling hydrocarbon structure to form lower boiling desired liquid fuel products of gasoline and light cycle oils which do not boil above a desired product range. The fluid catalytic cracking process (FCC) is the most widely used process to accomplish this purpose.
Crude oils and fractions thereof are normally subjected to pretreatment operations which remove arsenic and sodium to some considerable extent. Also, the heavy metals of nickel, vanadium, iron and copper, which tend to concentrate in the higher boiling portion of the crude oil boiling above about 1025.degree. F. or 1050.degree. F., may be removed partially by one or more methods comprising hydrogenation, delayed coking, solvent extraction and other operations known in the industry. For example, when hydrodesulfurizing the heavier high boiling portion of the crude oil, substantial metal contaminants are removed along with sulfur and nitrogen.
When subjecting the higher boiling portions of crude oil such as a vacuum resid without pretreatment to fluid catalytic cracking, the metal contaminants deposit on the catalyst, thereby reducing it's cracking activity. Other materials such as coke precursors providing Conradson carbon deposits including asphaltenes and higher molecular weight polycyclic structures of 2 to 4, or more, ring compounds break down, leaving deposit on the catalyst, thereby deactivating the catalyst. It has been observed that the heavy metals in the feed transfer almost quantitatively from the feed stock to the catalyst and are not economically removed therefrom, requiring replacement of metals contaminated catalyst with fresh catalyst.
It has been recognized by many in the industry that there is a substantial imbalance in the carbon/hydrogen ratio of the residuum portion of crude oil and such imbalance provides a complex set of technical and economic alternatives to be dealt with. This problem is aggravated substantially by changes in price available to the refiner for the asphalt content of crude oil when it becomes more valuable than refined liquid product such as gasoline, light and heavy liquid fuel oils.
The heavier crude oils are characterized as having a higher concentration of residuum. This residuum portion boiling above vacuum gas oils has a high concentration of nitrogen, sulfur, asphaltenes and higher boiling polycyclic ring compounds including porphyrins, as well as metal contaminants herein identified. A fundamental result of these increased heavy oil residuum components is a lower hydrogen to carbon ratio. However, product demand varies with the seasons and has been directed to providing more saturated middle distillate light oil products including materials readily converted to jet fuels during certain seasons which necessarily requires a substantially higher hydrogen to carbon ratio than is generally available from all residual oil fractions.
In a recent paper entitled "Hydrogen Utilization In Residuum Conversion", presented by Rosenthal et al, Chevron Research Company, at the 48th Midyear Refining Meeting, Session on Heavy Oils Processing, Tuesday, May 10, 1984, Los Angeles, Calif., reference is made to information developed by B. E. Stangeland of Chevron Research, concerned with the variation of hydrogen to carbon ratio with increasing molecular weight of crude oils and hydrocarbon fractions thereof. A chart developed by Stangeland shows the extent to which the carbon number must be reduced and sufficient hydrogen added to generate a desired light feed stock. This illustrates the importance of hydrogen addition when producing mid-distillates gas oils and lube oils. Such materials are characterized by quite low aromatic levels and desirably of high hydrogen content. The impact of using four different primary residuum processing steps on the hydrogen content of the raw liquid products obtained is graphically shown in FIG. 2 of the paper. The four processing steps chosen to demonstrate by comparison the concept were delayed coking and fluid coking (thermal processes) and FCC (fluid catalytic cracking) and residuum hydrodesulfurization as examples of catalytic processes. FIG. 2 clearly shows that the thermal processes produce lighter oils. However, these lighter oils are also much lower in hydrogen content and less than that desired in a middle distillate product fraction. Fluid coking offers the production of more liquid, but the liquid is of a lower hydrogen content than that obtained from delayed coking.
A fluid catalytic cracking operation is identified as producing high conversions, but yields products with relatively low hydrogen content. A residuum desulfurization (RDS), on the other hand, produces relatively light products that have a relatively high hydrogen content when obtained at lower residuum conversion levels.
The information above identified in the referenced paper shows indirectly and directly that a refiner has the choice of adding hydrogen to the product obtained at high yields in one or a combination of heavy oil pretreating and hydrofining steps bordering a primary conversion step of catalytic cracking.
The combination operation of the present invention and method of utilization is concerned in substantial manner with improving the hydrogen to carbon ratio of products of fluid catalytic cracking.